A hereditary syndrome involving a progressive atrioventricular conduction defect has been described in three generations of a large family. Clinical findings include a slow heart rate, episodes of lightheadedness, syncope, and absence of predisposing acquired or hereditary cardiac disorders. Electrocardiographic recordings reveal prolonged A-V conduction with usual onset at approximately age 30, loss of R-waves in the right precordial leads, arrhythmias which may occur only as a late manifestation. Progression from first degree to complete heart block is usually slow but occasionally a fulminant course may lead to sudden death within two to three years. Specific differences from other forms of conduction defects suggest that this family represents a new hereditary syndrome. We plan to study this syndrome through detailed medical genetic and cardiac investigations. The genealogy has been documented by personal interviews with family members and by means of questionnaires completed by individuals who reside in distant areas. Medical histories have been verified by physician and hospital records. A cardiologic objective is to determine the genesis and developmental aspects of the syndrome by means of specialized cardiopathologic techniques and such specific diagnostic measures as coronary arteriography, Bundle of His recordings, and metabolic studies to be performed in: 1) family members with heart block; 2) those at risk for heart block; 3) those who are known to be free of risk for heart block; 4) spouses of members of groups 1-3; and 5) individuals with heart block of etiology other than that of the syndrome under study.